Converting NSArray Contents to a varargs (With ARC) For Use With NSString initWithFormat

Question

We have some code today that takes an NSArray and passes it as a argument list to -[NSString initWithFormat:arguments] and we\'re trying to get this to work with ARC. Here\'s th

Answer 1

Expanding on @mcfedr's answer, this Swift 3 helper does the job:

import Foundation

@objc (FTStringFormat) public class StringFormat: NSObject {
    @objc public class func format(key: String, args: [AnyObject]) -> String {
        let locArgs: [CVarArg] = args.flatMap({ (arg: AnyObject) -> CVarArg? in
            if let arg = arg as? NSNumber {
                return arg.intValue
            }
            if let arg = arg as? CustomStringConvertible {
                return arg.description
            }
            return nil
        });
        return String(format: key, arguments: locArgs)
    }
}

Calling from Objective-C:

[FTStringFormat formatWithKey:@"name: %@ age: %d" args:@[@"John", @(42)]]

For the %@ format specifier we're using Swift's CustomStringConvertible protocol in order to call description on all of the array members.

Supporting all number format specifiers like %d and %f is not really possible because the NSNumber object doesn't reveal if it's an integer or float. So we could only support one or the other. Here we use intValue, so %d is supported but %f and %g are not.